In the conventional color halftone, as, for example, in a color television picture, a continuous family of colors is simulated by the superposition or complementary juxtaposition of three "single color" halftones or "separations". These three colors are often referred to as the three "primaries". For example, there may be a red separation, a blue separation, and a green separation.
In magazine or color print lithography, different primary color dots are deposited on the paper or other medium in various sizes or diameters so that the human eye integrates these dots and interprets them as various colors depending upon the dot mix. In photography, color negatives are provided which detect the various colors passing through the lens in a reversal process and, when printed, the various colors become viewable by the human eye. In all of these instances, however, the color perceived is due to either the reflective, transmissive, absorptive and/or radiant characteristics of the media (e.g., toners, inks, dyes, phosphors, pigments, etc.) involved--not the dispersive or diffractive properties.
According to the present invention, a full color image is created and is viewable in white light without the aid of colored inks, toners, sources, guns, or phosphors. A full color reproduction of a color picture, image or scene is realized by means of diffraction techniques. Instead of small, colored dots ("colored" by virtue of their reflective, absorptive, or radiant qualities) the halftone dots here produce their color by means of diffraction. The "dots" in this case are small "microgratings" whose grating spatial frequency has been chosen to diffract a particular portion of the visible spectrum in the direction of the viewer. Using additive primary methods, synthesis of any arbitrary color, (not necessarily a pure spectral color) is possible when the spectra from a plurality of different diffraction gratings are combined in the same plane.
The term "primary grating" refers to grating areas with the same spatial frequency. These "primary gratings" reproduce the red, green, and blue portions of the original image, and may be spatially combined in a number of ways. For example, analogous to color television techniques, a third of the area can be apportioned for the red dots; a third for the green, and a third for the blue. The dots/screens do not overlap. This will be referred to as a "complementary" screen design--one requiring a specially designed halftoning contact screen.
As in conventional color lithographic processes, however, here too it may be possible to allow the gratings to overlap or superimpose just as halftone dots overlap. This approach obviates the need for a special halftoning contact screen--and thus permits the use of conventional color separation methods. Again, just as in conventional color halftone printing methods, dot size will determine the net tone value of that particular halftone cell or dot.